The present invention relates to the field of thickness measuring devices, and more particularly to non-contact thickness measuring sensors such as a non-contact sensor for measuring the thickness of a sheet of calendered material.
In many fields of industry and science, it is desirable or necessary to measure the thickness of a dielectric layer or other sheet material overlying a substrate, without physically contacting either the sheet or the substrate.
One such thickness measuring device for use only with an electrically conductive substrate is disclosed in U.S. Pat. No. 3,243,992 to W. W. Woods, which is incorporated herein by reference. In the device disclosed in this patent, a cylinder is disposed perpendicular to a dielectric layer, with the open end of the cylinder spaced by a relatively short distance from the layer surface. The column of air within the cylinder is periodically pressurized with a vibrating loudspeaker-type diaphragm mounted over the end of the cylinder opposite the dielectric surface. The velocity of the pressurized air is measured as the air moves toward the open end of the cylinder. Since this velocity is related to the distance between the surface of the dielectric layer and a reference plane defined by the open end of the cylinder adjacent to the surface of the dielectric layer, the air velocity provides an indication of the separation distance to be gauged.
Woods' device also determines, using a mutual inductance gauge, the separation distance between the reference plane and the interface of the underlying conductive substrate with the dielectric layer. The operation of the mutual inductance gauge is explained in the Woods patent. As disclosed in that patent, the mutual inductance gauge includes a primary coil disposed adjacent to the cylinder on the reference plane defined by the open end of the cylinder, and a secondary coil magnetically linked to the primary coil by a mutual inductance factor which is proportional to the spacing between the primary coil and the substrate/dielectric interface. Consequently, the secondary coil output is indicative of the distance separating the primary coil, or reference plane, from the substrate surface at the interface with the dielectric layer.
As just explained, in the Woods device, the air cylinder gauge measures the distance from the reference plane to the exposed surface of the dielectric layer, while the inductance gauge determines the distance between that same reference plane and the interface between the conductive substrate and the overlying dielectric layer. By determining the difference in distances measured by these two gauges, Woods' thickness measuring device can determine the thickness of the dielectric layer. However, according to Woods' disclosure, the air cylinder gauge and the inductance gauge are disposed adjacent to each other along the surface of the layer whose thickness is to be measured. As a result, the accuracy of the overall device requires that both the air cylinder gauge and inductive gauge measure the respective distances from the same reference plane. Unfortunately, however, in many practical situations, it may be extremely difficult, if not impossible, to precisely align the inductance gauge with the reference plane of the air cylinder gauge. Alternatively, the substrate may have some intended or inadvertant curvature. Since the air cylinder and inductance gauges do not measure the respective distances at the same location, such misalignment or substrate curvature may lead to substantial error in the thickness measurement.
Moreover, the thickness measuring device may be subject to vibrations such that, at any instant during its operation, the air cylinder may be relatively closer or further away from the surface under test than the inductive gauge. As a result, error will be induced in the resulting measured thickness, since, as previously mentioned, the operation of the device assumes that both the inductance gauge and air cylinder gauge sides of the device are exactly disposed along the same reference plane, and that the substrate remains parallel to the reference plane.
Also, because one of the distance measuring gauges utilizes an electromagnetic coil to determine the distance from the reference plane to the substrate, Woods' apparatus is limited to use with electrically conductive substrates. Accordingly, such a thickness measuring device may not be completely satisfactory under many circumstances where the substrate is nonconductive.